The present invention relates to control schemes for electric machines, and, more particularly, to an improved control scheme for permanent magnet machines.
Due to their distinct characteristics, but also because of improvements in and reduced cost of permanent magnet (PM) technologies, PM machines are being used in an increasing number of applications, such as electrical propulsion systems for vehicles. Some applications, like electrical propulsion, require a wide operating range above the motor base speed, i.e., a wide range of flux weakening operation. Until recently, surface-mounted permanent-magnet synchronous (SMPMS) machines have been considered generally unsuitable for effective, wide-range flux weakening operation and there have been difficulties in achieving stable operation at high speeds.
With SMPMS machines, a known approach to flux weakening operation is to calculate the magnetizing current reference from SMPMS machine equations, with the assumption that all machine parameters are known. Limits for the magnetizing and torque current in the flux weakening region are calculated according to the presumption that an SMPMS machine operates within the voltage or voltage current limits. However, this approach is very sensitive to uncertainties related to the system parameters and does not offer stable performance under all conditions.
Recently published flux weakening schemes by Maric et al., xe2x80x9cTwo Improved Flux Wakening Schemes for Surface Mounted Permanent Magnet Synchoronous Machine Drives Employing Space Vector Modulationxe2x80x9d, IEEE IECON, Vol. 1, pp. 508-511, IEEE 1998 (hereinafter xe2x80x9cMaric 1998xe2x80x9d), xe2x80x9cRobust Flux Weakening Scheme for Surface-Mounted Permanent-Magnet Synchronous Drives Employing an Adaptive Lattice-Structure Filterxe2x80x9d, Conf. Proceedings IEEE APEC""99, pp. 271-276, IEEE 1999 (hereinafter xe2x80x9cMaric 1999xe2x80x9d), and Sudhoff et al., xe2x80x9cA Flux-Weakening Strategy for Current-Regulated Surface-Mounted Permanent-Magnet Machine Drivesxe2x80x9d, IEEE Transactions on Energy Conversion, Vol. 10, No. 3, pp.431-437 IEEE 1995 (hereinafter xe2x80x9cSudhoffxe2x80x9d), offer important advantages in that the schemes do not use machine parameters for calculations in the flux weakening region.
The Sudhoff control scheme is used in the flux-weakening operation of an SMPMS drive. The control scheme uses an error in the torque controlling current component to generate the required demagnetizing current. However, the controller relies on the hysteresis-type current controller, which is not suitable for digital implementation. Also, stable operation over a wide speed range is not practicable.
The Maric schemes are based on the Sudhoff scheme, but modified to use the current controllers in the synchronous reference frame. The Maric 1998 control scheme operates in a wide speed range, but requires non-zero torque control error, which is not suitable for torque-controlled drives used in some applications, such as electric/hybrid vehicle applications. Similar to the Sudhoff scheme, the Maric 1998 scheme detects the steady-state error in the torque current regulation, and then uses the error to generate the magnetizing current reference. In contrast, the Maric 1999 scheme uses closed-loop control of the phase voltage magnitude to generate magnetizing current reference for the flux-weakening operation. The Maric 1999 method is relatively robust, without steady state error present the Maric 1998 and Sudhoff schemes. However, it is more computationally complex than the Maric 1998 scheme. Also, the response of the Maric 1999 control algorithm to sudden torque changes is slow, and can even become unstable due to the loss of current control. The Sudhoff and Maric 1998 algorithms have faster transient responses to sudden torque changes, but require constantly present error in the torque current regulation.
Choi et al., xe2x80x9cDesign of Fast Response Current Controller Using d-q Axis Cross Coupling: Application to Permanent Magnet Synchronous Motor Drivexe2x80x9d, IEEE Transactions on Industrial Electronics, Vol. 45, No. 3, June 1998, pp. 522-524 (hereinafter xe2x80x9cChoixe2x80x9d) describes another SMPMS machine control scheme. In the Choi scheme, a term dependent upon the torque controlling current is used to generate a temporary reference for the magnetizing current. Although this control approach can speed-up the torque transients below base speed, no control strategy for the flux weakening is described. Moreover, the Choi control scheme does not provide for drive operation above the base speed.
Accordingly, there exists a need for an improved control scheme for SMPMS machines that offers fast transient responses and a wide operational range of speed.
It is an advantage of the present invention to provide a machine control scheme that improves drive performance in a wide speed range, and enables SMPMS machines to be used in applications where fast transient response is required, for example, in motor/generator applications for electric or hybrid vehicles.
According to one aspect of the present invention, a control scheme for an SMPMS drive can use a combination of an open-loop magnetizing current reference calculation and a stabilizing feedback term, which speeds-up the torque transient response. The feedback term increases the stability margin during torque transients by increasing the available voltage margin for current control. The magnetizing current reference calculation takes into account the saturation effects in the SMPMS drive, which occur at peak torque points, and compensates for them. By taking into account saturation effects, stable operation at high speed is achieved, thereby increasing the speed range of the SMPMS drive.